It is proposed to develop theoretical models which relate the rheology of blood in microvessels to the mechanical properties of individual red blood cells. The rheological properties of blood are crucial both to the role of the circulation in mass transport to and from tissues, and to the mechanical load on the heart resulting from peripheral resistance. Because normal red cells are highly deformable, blood flows readily through the smallest vessels in the microcirculation, whose diameters are less than that of undeformed red cells. The elastic and viscous properties of red cells have been much studied and are now fairly well established. This allows the development of detailed quantitative models describing the deformations of red cells in microvessels, and predicting the resulting microvascular rheological properties of blood. The focus of the proposed research will be flow in capillaries in which red cells flow in single file or in two files. Previous models for single-file flow have generally assumed axisymmetric cell shapes. We propose to develop models showing the consequences of non-axisymmetry in single-file flow, including the phenomenon of "tank-treading". In two-file flow, a staggered arrangement of cells called "zipper" flow is frequently observed. The rheological consequences of tank-treading are potentially important in this flow, and will be studied. Techniques developed in these studies of flow in uniform vessels will provide a basis for modeling the motion of red cells in diverging capillary bifurcations. Although the proposed work is theoretical, it relies heavily on interaction with experimental hemorheologists. In pursuing these studies, the principal investigator intends to continue and extend his existing collaboration with Professor P. Gaehtgens (Berlin, West Germany), who is a leader in this field. Experimental observations of Dr. Gaehtgens and other hemorheologists form the starting point for most of the proposed studies, and as the work proceeds the results and predictions will be regularly compared and tested against available experimental data.